Optical instrument



Patented July 22, 1924.

UNITED STATES PATENT OFFICE.

ROBERT W. WILLSN', OF CAMBRIDGE, MASSACHUSETTS; ARTHUR H. BROOKS, ADMIN- i ISTRATOR- OF SAID HUBERT W. VIILLSON, DiECEASED. i i

OPTICAL NSTRUMENT.

Application filed November To @ZZ whom t may concern.:

Be it known that I, Ronun'r W. Winnson, a citizen oi the United States, residing at Cambridge, in the county olI ).liddlesex and State of Massachusetts, have ini'fented certain new and use'tul Improvements in Op-l ti `Aal Instruments, Yfully described and represented in the following specification and the accompanying drawings, forming a part ot the saine.

This invention relates to optical instruments and has for an object to provide an optical instrumeiit which may accurately he directed upon the center of a circular or apprcximately circular object.

An optical instrument embodying the 'inf rention facilitates the measurements of anguiar distances trom the center ot the circular or approximately circular objects, such as the suns disk, the tull moon, or other celestial or terrestial objects ot' circu lar or nearly circular form.

While the neuT optical instrument is 'susceptible oii many diti'erent uses it may most. conveniently loe described in connection With its use in measuring the altitude and bearing oli the center ot the suns disk.

Surveyors transits, theodolites, altazimuths, and other instruments heretofore customarily used in measuring these coordinates are provided with telescopes Whicn procure an image of the suns disk in the iorm ot' a circle of comparatively large diameter and containing nothing to indicate its center. To obtain altitude and loearing readings from the sun by means of such instruments it is necessary to set the instrument so that the circular image ot the suns disk is tangential to one ot the two cross wires or hairs ot the instrument. To obtain looth altitude and horizontal hearing readings, therefore, two settings upon the sun must be made and each reading must loe corrected for the semidiameter of the suns disk at the time the observation is taken.

tu instrument constructed in accordance with the present invention produces an iniage ot the sun which is circular or approximately so and which contains at Vits center a minutie spot which is eitherfbrighter or darker than the rest ot' the image. If the instrument is provided With the usual cross Wires, the central spot of the image may isiaicasdbyths une fin rig. i;

23,'1921. Serial No. 517,176.

accurately be set at the intersection of the cross Wires. The instrument Will then be directed accurately at the center of the solar disk. By a single setting of the instrument upon the sun, therefore, both altitude and horizontal bearing readings may be obtained, and neither of the'readings Will require correction for-fthe suns semi-diameter.

An optical instrument embodying the invention includes an optical element co1nprising a convex lens, Which'may he similar to the object glassiof the 'telescope of a surveyo1"s transit, and a conical prism in front of the lens and coaxial therewith. The instruinent is most desiralolyl also provided `with an eye-piece of any usual construction and with cross wires or hairs such as used `in a surveyors transit and in other types of telescopes. The Wedge angle of the conical prism is so proportioned `to the index of refraction of the glass of Which the prism is made that light striking the prism in a direction parallel to its axis is deflected at an angle which is approximately equal to the apparent semi-diameter of the suns dislr or the semi-diameter of any other circular or approximately circular object upon which the instrument is to be directed.

In the accompanying` drawings, have, tor the salte of illustration, shown the invention embodied in a telescope ofthe type customarily used in a surveyors transit. in the drawings I Fig. l is a longitudinal section of the instrument 5 Fig. 2 is a transverse section on the line Q-Q of Fig. l on a larger scale;

Fig. 3 is a sectional diagrammatic 'View upon an enlarged scale, showing the prism and the 'object glass; j

Fig. l is a magnified view ot the image of a circular object :tormed by the object glass,

Without the prism, in the plane of the focus .of the object glass which is indicated loy the line ie-5 in Fig. 1 Fig. 5 is a magnified `vievv of the image oli a circular object having anse! l I eqpu the Vangle of' deviation of il'joruined2b;` theprisin and obi ect lane of thefocus l'thefo iectM vvil Fig. 6 is a' magnified vievv of the image of a circular object having a semi-diameter equal to the angle of deviation of the prism, formed bythe prism and object' glass onl a plane slightly withinthe focus off the object glass, which is indicated by the line 6.-6Vin. Fig? l;

Fig. 7 is a magnified view of the image of a circular object having a semi-diameter equal to the angle of deviation of the prism, formed by the prism and object glass in a plane slightly without the focusof thev objectglass, represented by the line 7 7 in Fig. 1;

Fig. 8 is a sectional view of a modified form of prism; and

Fig. 9-i is a sectionalV view showing' the modification iny which the prism and lens are formed froma singlepiece of glass.

In the form shown inthe drawings the new instrument is likethe telescope customk arily usedy in the surveyors transit except that' a conical prism is removably mounted in frontV of the object glass of the telescope.

The object glass 10,- a convex lens, is mounted in a setting- 11 at." the outer end of the object glass tube l2y which is slidably mounted in the telescope tube 13. The object glass tube may be moved longitudinally with respect to the telescope tube by means of' aV pinion- 14 and a rack 15. In the telescope tube are mounted cross wires 16 which intersect in the axis A of the object glass, and an eye-piece 17 wliichinay be focused uponk the plane of the cross wires. It may be noted that the expression crossA wires as used in this application, including t-he claims, is to be understood in al general sense, and not as a limitation upon the mater'ial from which the cross Wires are made. This material may be metal, hair, spider web, or any other material usable for this purpose. As the features just described are commonly usedin the telescopes of. sur.- veyors" transits and arel well understood, they are not described in detaill. It will'be understood also that the telescope may be provided witha mounting which permits it to be turned` inl horizontalf and vertical planes, and' that its mounting may be provided, as is customary, with a level, a compass, and with scales by which readings of the bearing and altitude of an object upon which the telescope is directed may be taken.

A conical prism 20 is removably mounted in front of the object glass 10; For this purpose a short tube or ring 21y is mounted upon the outer end of! the object? glass tube 12. The inneredge of the ring: 21 sets against the shoulder 2-2, whichis customaril'y provided upon the object glass tube 12. The ring, 21 contains a longitudinal slot 23 near its inner end? so that the ring may beY clamped upon the object glasstube 12 by means of a-screw 25 passing through a split lug 24 as best seen in Fig. 2. The set ting of the prism 2O comprises an outer member- 26 which has a smoothrunning fit inthe outer portion of theV ring 211, and an inner member 31 which is screwed into the outer member 26. The prism 20 is held bretween an internal annular flange 29 formed on the outer member 26' of the setting and the lower edge of the inner member 31 of the setting. The outer member 26 has at its outer endl anexternal flange 27 which has a smooth surface 28 which contacts with the outer end of. the ring 2l. In the outer surface of the outer member 26 ofthe setting is formeda groove 33 into which enters the end of! a screw 32 carried by the ring 21. The engagement between the groove 33 and the screw 32 retains the setting in the ring 21but permitsthe settingl to be turned through anangle of 180O (see Fig. 2).

The mounting described.' is designed to retain the prism 2O with its axis coincident with the axis A of the object glass. The provisionl for turning the prism through 180 permits the correction of errors which may'arise by slight inaccuracies in forming the prism or the mounting so that the prism axis is not exactly coincident with that of the lens.l

As best seen in Fig. 3, the prism-20 has a conical surface 35. The opposite orl base surface 36of the prism is a plane surface perpendicular to the axis of the conical sur face. For convenience in mounting, the conical surface 35 is not extendedl until it meets the base surface 36of the prism, but

is cut off so that a cylindrical peripheral surface` 37 is provided; In order that the prism may be accurately mounted it is important that the periphery of the base surface 36 lie ina plane perpendicular to the axis ofthe conical surface 35. It is not essential, however, that the entire base surface 36V lie in` this: plane as a base surface may iff desired be made spherically curved as hereinafter. pointed: out.

It isapparent that rays of light striking the prism shown in. Fig. 3 in a direction parallely to the axis of the prism will be deflected towardi the axis as they pass through the prism. The angle a of deflection of' such rays, whichA may be called the angle of deviation of the prism, depends upon the index, of refraction of' the glass, or other material of which the prism is made, andthe wedge angle ofi the prism. The wedge angle isthe angle between the conical surface 35 and a plane perpendicularfto the axis-of this surface. The relation between the angle ofV deflection or deviation a andthe wedge angle b may be computed by known methods. Vhen the angles are both small, this relation may for practicalapurposes be obtained from the following formula a: (it-1) X?) where a represents the index of refraction of the material of the prism and b the Wedge angle ot the prism.

For a reason hereinafter explained, the Wedge angle o of the prism is so proportioned to the index of retraction of the ma terial ot the prism that the angle of deviation a is approximately equal to the apparent semi-diameter of the circular object upon the center of which the instrumentu is to be set.

The particular instrument shown in the drawings is designed to be set upon the center ot the suns disk and the prism is so proportioned that the angle of deviation a equals the mean semi-diameter ot the suns dist: or o 1G 2". "With glass of an ,index oi retraction ot 1.523 this angle of deviation is obtained by making the Wedge angle Z) ot the prism 0 30 39.4. For the salte ot' clearness, however, the size of the wedge angle Y) is considerably exaggerated in the drawings.

The operation ot the instrument described i as Afollows:----.lt will be understood that ii' the prism is removed and the telescope directed toward the sun the rays of the sun which are substantially parallel when they strilte the object glass l() ot the telescope `.vill be brought together so as to torni an image of the sun at the focus of the object glass. Such rays are indicated by the dot-anddash lines l0 in Fig. l. The plane of the focus is represented by the line #l-- in Fig. l. The focal plane is there shown as the plane in which the cross Wires 16 are mounted in the telescope. By moving the object glass tube inwardly or outwardly by means of the gear l-l and "ack l5, however, the focal plane 4-5 may be offset .in either direction from the plane of the crosss Wires 16. The image 91 of the sun termed in the focal plane by the object glass alone is shown in Fig. It consimply oi a circle of fairly large dianiieter containing nothing to indicate the center et the circle. It the 'focal length of the object glass is 8 inches, the diameter of the image shown in Fig. 3 Will be about 0.0i' inch, although it will, ot course, appear much larger When viewed through the eyepiece oitl the telescope.

lilith the prism 2O in the position indicated in Fig. l it is apparent that when the instrument is directed at the sun the rays ot the sun indicated by the dotted lines 4l in l) which are parallel When they strike the prism Q0 will be deflected in- VWardly so that they are convergent When they strike the object glass l0. It the angle ot deviation ot the prism is precisely equal to the semi-diameter of the sun at the time ot the observation, the image ot the sun formed in the focal plane 4*--5 of the lens will have the appearance oit a circle of twice the diameter of the image termed by the lens alone. This image I, shown in Fig. 5, is really a composite image formed by the superposition of an iniinite number of circular images of the same size as the image z shown in Fig. 4, and displaced from the center' ot the composite image by a distance equal to their semi-diameters. Some of the component images are `indicated by the dotted lines z" in Fig. 5, although it should, of course, be understood that these component images are not seen separately.

The image I formed by the lens and prism upon a plane 6---G slightlywithin the itocns ot' the object glass has the torni illustrated in Fig. G. This image I, like that shown in Fig. 4, is really a composite image formed by the superposition oi' an infinite number of component circular images z" of substantially the same diameter as the image shown in Fig. 4L, but in this instance the component images a" are displaced lrom the center of the composite image by distances slightly lessthan their semi-diameters, so that the component images overlap slightly at the center of the composite image. As the result, the composite image l1 shown in Fig. 6 appears as a circle oi slightly less than twice the diameter of the iniagc z' shown in Fig. a and having at its center a minute bright spot C.

The image I formed by the prism and object glass in a plane 7 7 ust Without the focal plane of the object glass has the form illustrated in Fig. 7 In this case the component circular images z" are displaced 'trom the center oi' the composite image by a distance slightly greater than their semi-dia meters. As a result, the composite image I appears as a circle ot slightly more than twice the diameter than the image shown in Fig. 4 and having at its center a minute black spot C.

It Will be understood that an image like that shown in Fig. 6 is formed in the focal plane of the. object glass if the angle of deviation of the prism is slightly greater than the semi-diameter of the sun at the time of the observation, and that an image like that shown in Fig. 7 is iormed in the iocal plane if the angle of deviation of the prism is slightly less than the semi-diameter of the sun at the time ot the observation. Owing, however, to the seasonal variation in the diameter of the suns disk, due to the variation in the distance between the earth and the sun, it has been Yfound most convenient to malte the angle of deviation of the prism precisely equal to the mean semi-diameter oit the sun and to obtain the bright or dark spot at the center oi the image by observing the image on the plane displaced from the focal plane When the semi-diameter of the sun at the time of the observation is just equal to its mean semidiameter.

The eye-piece of: the telescope may be focused upon the plane ofj the cross wires. 16 so that the image which is observed through the eye-piece is that formed inthe plane of the cross wires. By moving the object' glass tube inwardly or outwardlythe focal plane of the object glass may be shifted with` respect to the eye-piece andthe cross wires so that the planeof the image observedi may be either withinor without the focal plane.

It is, therefore, a very simple matter to set the instrument accurately upon the ceuter ofthe sun. To dofthis it is merely necessary to focus the eye-piece upon the plane of the cross wires, direct the instrument. toward the sun, andv then. move the object glass tube inwardly or outwardly by means of the pinion lll until the image observed contains a minute bright or darli central spot. The telescope is then turnedluntil' this spot lies at the intersection of the cross. wires 16. rIlhis indicates that the axis of the telescope is pointed directly at the center of the sun. If the telescope is mounted in the manner customary in surveyors transits, the altitude and bearing of the. sun may then be read ott atv once andv no correction for the semi-diameter of the sun need be applied to these readings.

When very accurate results are required, it is desirable to eliminate any slight error which may resulty from slight inaccuracy in the making of the prism. In order to eliminate errors from this cause it is merely necessary to take an observation with the prism setting 26, 3l turned so that one end.y ot the groove 33 is in engagement with the screw 32, and another observation with the prism setting 26, 3l turned so that the other endrof the groove 33 is in engagement with the screw 32. As the prism is turnedV throngl anV angle of 180@ between the two observations any error caused by inaccuracy in the positioning of the prism axis in the first cl servation willbe exact-ly reversed in the sece ond observation, so that the true position of the sun may be ascertained by merely taking the mean between the results of the two observations.

In what' has been said it has been assumed that the solar disk observed is truly circular. The instrument may, however, be used in substantially the same manner when the suns disk appears somewhat elliptical because of atmospheric refraction. In this case the images I, and I will" be elliptical and the component images i. will be ellipses, instead of circles, and in consequence the central spot C or C will be elongated instead of round. By observing the image in the proper planes, however, the central spots may bemade so small'that they appear prac tically as points and so may be accurately set; at the intersection of; the crosshairs;

lhen it is desired to take, observations up on objects. other than'l the sun, the prism 20 audits mounting may readily beremovedby loosening the screw and'slipping the vring 2l; olf` the end ofthe objectV glass tube, 12.

Many modihcations may be madey in the device described. withoutv departing from my invention. rllhus when the base off the prism is made flat as shown in Figs. 1'; and 3', the pris-m may if/ desired be located with its fiat side turned outward. The illustrated mounting ofthe prism may bechanged, and iii-'case the prism` is made with great accu'- racy, or in case very accurate readings are not required, provision for turning the prism through an angle of 180O is not necessary. I shall not, however, attempt to catalogue all changes whichmay be made in the em bodiment described without departing from my invention, butv I will mention a number of changes which in themselves constitute part' of my invention and which serve to adapt the instrumentV to various different uses.

Then the conical surface of the prism is placed'outwardv it is not essential that base surface of the prism be a plane surface, and it: may desirably under certain circumstances be made spherically curved. I have found that when the prism is used in combination with an object glass of very high power it is desirable to make the base surfacey of a prism slightly concave so that the prism has the form` illustrated in Fig. 8. This has the effect of'producing a sharper image in the focal plane of the object' glass. The desirable curvature of the base surface is, however, somewhat exaggerated inA Fig.' S for the salie of clearness. Y

Futhermore, in the case of an instrument which is to be used only for solar observations an ordinary object glass may be dispensed with and the base surface of the prism made convex as illustrated" in Fig. Si, so that a single piece of glass, or, if pre. ferred, a plurality ofI pieces of glass secured together without air spaces between them may be made to perform the function of both the prism and the Object glass in the speciic instrument described. It should be understood that in the claims which follow the mention of' a lens and a conical prism is intended to include a lens and prism formed of a single piece of glass as shown in Fig. 8, as well as a separate lens and prism, except in such instances as a prism'separate from the lensis expressly specified.-

For obtaining altitude and bearing, readings upon the moon, or other circular or approximately circular object, avprisni may be provided which has an angle of deviation approximately equal to the semi-diameter of the moon or other object to be observed.

I claim as my invention:

1. An optical instrument, comprising a tube containing cross wires, and an optical element mounted at the outer end of the tube and formed to deflect parallel rays from a circular object so as to produce in the plane of the cross wires a circular image with a spot at its center having a diiferent degree of illumination from the rest of the image.

2. An optical instrument, comprising a tube, an o tical element mounted at the outer end of said tube and comprising a convex lens and a prism having a conical sur face whose axis is substantially coincident with the axis of the lens, and cross wires in said tube intersecting in the axis ofthe optical element.

3. An optical element, comprising a convex lens and a prism having a conical surface whose axis is substantially coincident with the axis of the lens.

l. An optical element, comprising a convex lens and a prism separate from the` lens and having a conical surface, the prism being mounted in front of the lens and with its axis substantially coincident with that of the lens.

5. In an optical instrument adapted to be set upon the center of a circular object, an optical element comprising a convex lens and a prism having a conical surface substantially coaxial with the lens, the prism having an angle of deviation approximately equal to the apparent semi-diameter of the object to be observed.

6. In an optical instrument adapted to be set upon the center of the suns disk, an optical element comprising a convex lens and a prism having a conical surface substantially coaxial with the lens, the prism having an angle of deviation approximately equal to the semi-diameter of the sun.

7. An optical instrument adapted to be set upon the center of the suns disk, com prising a convex lens, a. prism having a conical surface substantially coaxial with said lens and having an angle of deviation equal to the mean semi-diameter of the sun, and means for observing the image of the sun formed by said prism and lens in the focal plane of the lens and in planes displaced from the focal plane of the lens.

8. A n optical instrument adapted to be set upon the center of the suns disk, comprising a telescope tube containing cross Wires, means for observing an image formed in the plane of said cross wires, a convex objectglass located with its axis passing through the point of intersection of said cross wires, a separate conical prism substantially coaxial with said object glass and having an angle of deviation equal to the mean semi-diameter of the sun, and means for moving said object glass axially so as to locate the focal plane of the object glass in the plane of said cross-wires and to displace the focal plane from the plane of said cross wires.

9. In combination with the telescope of a surveyors transit or similar instrument, a conical prism secured in front of the object glass of a telescope with its axis substantially coinicident with that of the object glass.

10. An attachment for the telescopes of surveyors transits and like instruments, comprising` a conical prism having an angle of deviation approximately equal to the semi-diameter of the sun, and a setting for said prism removably secured to the telescope and arranged to position the prism in front of the object glass of the telescope with its axis substantially coincident with that of the object glass.

11. In combination with a telescope of a surveyors transit or similar instrument, a prism secured in front of the object glass of the telescope having an outer conical sur face whose axis is substantially coincident with that of the object glass, and an inner base surface which is concave, spherically curved and coaxial with the object glass.

12. VAn optical instrument adapted to be set upon the center of the circular object, comprising a tube, a convex lens mounted near the outer end thereof, a separate conical prism, and a mounting for said prism arranged to hold it infront of said lens with its axis substantially coincident therewith, said mounting of said prism being adapted to permit the turning of the prism through an angle of 1800.

13. An attachment for the telescopes of surveyors transits and like instruments, comprising a conical prism, a setting for said conical prism, a ring in which said setting is turnably mounted, said setting and ring having cooperating stops adapted to locate the prism setting in two positions separated by an angle of 180", and means for removably securing said ring to the outer end of the telescope.

In testimony whereof I have hereunto set my hand in the presence of two subscribing witnesses.

ROBERT W. VILLSON. Witnesses:

A. MEAD WHEELER, E. WILLARD REIPPEN.

llO 

